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Rethink definition of pg_attribute.attcompression.

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Redefine '\0' (InvalidCompressionMethod) as meaning "if we need to
compress, use the current setting of default_toast_compression".
This allows '\0' to be a suitable default choice regardless of
datatype, greatly simplifying code paths that initialize tupledescs
and the like.  It seems like a more user-friendly approach as well,
because now the default compression choice doesn't migrate into table
definitions, meaning that changing default_toast_compression is
usually sufficient to flip an installation's behavior; one needn't
tediously issue per-column ALTER SET COMPRESSION commands.

Along the way, fix a few minor bugs and documentation issues
with the per-column-compression feature.  Adopt more robust
APIs for SetIndexStorageProperties and GetAttributeCompression.

Bump catversion because typical contents of attcompression will now
be different.  We could get away without doing that, but it seems
better to ensure v14 installations all agree on this.  (We already
forced initdb for beta2, anyway.)

Discussion: https://postgr.es/m/626613.1621787110@sss.pgh.pa.us
This commit is contained in:
Tom Lane
2021-05-27 13:24:24 -04:00
parent a717e5c771
commit e6241d8e03
29 changed files with 261 additions and 384 deletions
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30
doc/src/sgml/ref/alter_table.sgml
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@@ -104,7 +104,6 @@ WITH ( MODULUS <replaceable class="parameter">numeric_literal</replaceable>, REM
GENERATED { ALWAYS | BY DEFAULT } AS IDENTITY [ ( <replaceable>sequence_options</replaceable> ) ] |
UNIQUE <replaceable class="parameter">index_parameters</replaceable> |
PRIMARY KEY <replaceable class="parameter">index_parameters</replaceable> |
COMPRESSION <replaceable class="parameter">compression_method</replaceable> |
REFERENCES <replaceable class="parameter">reftable</replaceable> [ ( <replaceable class="parameter">refcolumn</replaceable> ) ] [ MATCH FULL | MATCH PARTIAL | MATCH SIMPLE ]
[ ON DELETE <replaceable class="parameter">referential_action</replaceable> ] [ ON UPDATE <replaceable class="parameter">referential_action</replaceable> ] }
[ DEFERRABLE | NOT DEFERRABLE ] [ INITIALLY DEFERRED | INITIALLY IMMEDIATE ]
@@ -391,24 +390,27 @@ WITH ( MODULUS <replaceable class="parameter">numeric_literal</replaceable>, REM
</term>
<listitem>
<para>
This sets the compression method to be used for data inserted into a column.
This form sets the compression method for a column, determining how
values inserted in future will be compressed (if the storage mode
permits compression at all).
This does not cause the table to be rewritten, so existing data may still
be compressed with other compression methods. If the table is rewritten with
<command>VACUUM FULL</command> or <command>CLUSTER</command>, or restored
with <application>pg_restore</application>, then all tuples are rewritten
with the configured compression methods.
Also, note that when data is inserted from another relation (for example,
by <command>INSERT ... SELECT</command>), tuples from the source data are
not necessarily detoasted, and any previously compressed data is retained
with its existing compression method, rather than recompressing with the
compression methods of the target columns.
with <application>pg_restore</application>, then all values are rewritten
with the configured compression method.
However, when data is inserted from another relation (for example,
by <command>INSERT ... SELECT</command>), values from the source table are
not necessarily detoasted, so any previously compressed data may retain
its existing compression method, rather than being recompressed with the
compression method of the target column.
The supported compression
methods are <literal>pglz</literal> and <literal>lz4</literal>.
<literal>lz4</literal> is available only if <literal>--with-lz4</literal>
was used when building <productname>PostgreSQL</productname>.
(<literal>lz4</literal> is available only if <option>--with-lz4</option>
was used when building <productname>PostgreSQL</productname>.) In
addition, <replaceable class="parameter">compression_method</replaceable>
can be <literal>default</literal>, which selects the default behavior of
consulting the <xref linkend="guc-default-toast-compression"/> setting
at the time of data insertion to determine the method to use.
</para>
</listitem>
</varlistentry>